It is proposed to examine the molecular basis for the action of the Escherichia coli biosynthetic L-threonine deaminase in regulating the expression of the isoleucine-valine enzymes. Threonine deaminase will be purified from specific genetic regulatory mutants and compared to the wild type enzyme with respect to chemical and physical properties, enzyme maturation, and tRNA binding. These purified enzymes will also be tested for repressor activity in a DNA directed in vitro protein synthesizing system, which is now under development, and the amino-terminal amino acid sequence will be determined using the automated system. Dihydroxyacid dehydrase, the ilvD gene product of the ilvADE operon, has been stabilized in crude extracts, and will be purified and characterized. Information about dihydroxyacid dehydrase will add to our fundamental understanding of several aspects of the expression of the ilvADE operon. It is also proposed to examine the rat liver phenylalanine hydroxylase to test for the presence of a self-regulating gene control system.